1. Field of the Invention
This invention relates to a circuit arrangement for monitoring the square of the RMS value of a periodic signal by means of a squaring element connected to an integrating element.
2. Discussion of the Prior Art
The equation for defining the square of the RMS value of a periodic signal u reads ##EQU1## i.e., the square of the RMS value is equal to the quotient of the integral of the square of the periodic signal taken over one cycle and the period T.
A circuit arrangement for measuring the RMS value of an AC voltage is described in German Auslegeschrift No. 1,935 544, in which squaring of the AC voltage is accomplished by supplying it to the two inputs of a multiplier, the output of which is connected to an integrator. The output variable of the integrator corresponding to the RMS value is fed, via a coupling element, to a dividing input of the multiplier. Such a circuit arrangement is relatively sluggish, so that it is not suitable for rapid monitoring whether the RMS value exceeds a limit in time spans comparable to the period of the periodic signal.
German Offenlegungsschrift No. 2 314 870 describes a circuit arrangement for measuring the square of the RMS value of periodic voltages or currents in which the periodic signal is fed to a squarer. The output signal of the squarer is sampled at equi-distant time intervals within a period, and the instantaneous values present at the instant of sampling are measured by fast voltage-frequency converters and are fed to a counter. Thus, a kind of numerical integration of the square of the periodic signal is performed during each period. This circuit arrangement furnishes a signal proportional to the square of the RMS value at the end of each period and thus works very fast. However, it requires not inconsiderable circuitry.
It is an object of the present invention to describe a circuit arrangement of the kind mentioned above, which provides an indication of when the square of the RMS value in successive periods of a periodic signal exceeds a limit a presettable number of times.